1. Field of the Invention
The present invention relates to an electromagnetic switch which turns on/off a conduction (energizing) current of a starter motor.
2. Description of the Related Art
Conventionally, an electromagnetic switch which turns on/off a conduction (energizing) current fed to a starter motor which starts an engine is widely known.
When a movable contact is driven by magnetomotive force which an exciting coil generates by being energized, so as to contact a pair of fixed contacts which is placed opposite the movable contacts, the pair of fixed contacts are short-circuited with each other. This makes the electromagnetic switch close an electric circuit. The exciting coil is conducted with a switch terminal which is energized in response to ON operation of an ignition switch, and the pair of fixed contacts are conducted respectively with a battery terminal and a motor terminal, both of them are formed protrusively outward at a contact cover fitted into a yoke of the exciting coil. A battery cable is connected to the battery terminal, and a motor lead is connected to the motor terminal. The electric circuit is closed by the operation of the electromagnetic switch, thereby a motor generates torque in response to power feeding from a battery. The torque is slowed down with an epicycle reduction gear for example, and is transmitted to a pinion gear. An engine crankshaft is rotated by engaging the pinion gear with a ring gear provided in the engine crankshaft, and an engine starts up.
In an electromagnetic switch disclosed in a Japanese Patent Application Laid-Open Publication No. 2004-319128, as shown in FIG. 1, collars 101, 102 are insert-molded at the bottom of a contact cover 100, and in each hollow part of the collars 101, 102, a battery terminal bolt 103 and a motor terminal bolt 104 are inserted respectively. The battery terminal bolt 103 and the motor terminal bolt 104 are bolt-like members having respective heads 103a, 104a in which fixed contacts 105, 105 are formed, and respective feet 103b, 104b in which male screws are formed on the surfaces. The collar 101 (102) extends from the inside of the contact cover 100 to the outside. One end of the collar 101 (102) abuts against a back of a side facing the fixed contact 105 of the head 103a (104a) inside the contact cover 100, and the other end of the collar 101 (102) abuts against a fixed washer 106 outside the contact cover 100. Ring terminals 107a, 108a are formed at a top of a battery cable 107 and a motor lead 108 respectively. The ring terminals 107a, 108a are respectively interposed between the fixed washers 106,106 and nuts 109,109 screwed on the male screws formed in the feet 103b, 104b of the terminal bolts 103, 104, then the ring terminals 107a, 108a are tightened.
By the way, in the case where the electromagnetic switch is used under a severe environment where excessive vibration from the engine is directly received, the nut 109 may loosen by excessive vibration. Therefore, it is necessary to prevent the battery cable 107 and/or the motor lead 108 separate from the respective collars 101,102. Then, if the battery terminal bolt 103 and the motor terminal bolt 104 are formed through the collars 101, 102 as the above composition, a tightening loads of the nuts 109,109 are received in the axial direction of the collars 101, 102 having high strength. Consequently, the nut 109 can be firmly tightened. In addition, looseness of the nut 109 by vibration or aging can be controlled since the creep tolerance of metal under compressive load is excellent compared with resin.
However, in the above electromagnetic switch, other than the above composition, an O-ring is fitted on an outer circumference of each head 103a, 104a of the battery terminal bolt 103 and the motor terminal bolt 104 in order to secure the airtightness in the contact cover 100. In order to improve sealability for airtightness, an O-ring with sufficient thickness should be adopted, compressed strongly, and as a result, largely elastically deformed. However, in such composition, increase in the thickness of the O-ring may require larger outer diameter of the contact cover 100. In order to avoid the enlargement of the outer diameter of the contact cover 100, thin sealing member should be used.